Abstract

Wind and warmth sensations proved to be able to enhance users' state of presence in Virtual Reality applications. Still, only few projects deal with their detailed effect on the user and general ways of implementing such stimuli. This work tries to fill this gap: After analyzing requirements for hardware and software concerning wind and warmth simulations, a hardware and also a software setup for the application in a CAVE environment is proposed. The setup is evaluated with regard to technical details and requirements, but also - in the form of a pilot study - in view of user experience and presence. Our setup proved to comply with the requirements and leads to satisfactory results. To our knowledge, the low cost simulation system (approx. 2200 Euro) presented here is one of the most extensive, most flexible and best evaluated systems for creating wind and warmth stimuli in CAVE-based VR applications.

Wind and warmth sensations proved to be
able to enhance users' state of presence in Virtual
Reality applications. Still, only few projects deal with
their detailed effect on the user and general ways of
implementing such stimuli. This work tries to fill this
gap: After analyzing requirements for hardware and
software concerning wind and warmth simulations, a
hardware and also a software setup for the application in a
CAVE environment is proposed. The setup is evaluated
with regard to technical details and requirements, but
also - in the form of a pilot study - in view of user
experience and presence. Our setup proved to comply with
the requirements and leads to satisfactory results.
To our knowledge, the low cost simulation system
(approx. 2200 Euro) presented here is one of the most
extensive, most flexible and best evaluated systems for
creating wind and warmth stimuli in CAVE-based VR
applications.

1.
Introduction

Enhancing presence is one of the most important topics
in Virtual Reality research. Recent work shows that a high
level of presence can be achieved, for example by a
reasonable combination of feedback modalities [FW13].
Due to technical advancement, virtual worlds have already
become more and more immersive. In this area, especially
concerning rendering techniques and spatial sound,
many advances have been made. But how can further
natural sensations, e.g. airflow during navigation
tasks, heat of a fire in a rescue scenario [MKB13]
or simply the warmth inside a simulated desert be
made perceivable in virtual environments? A common
type of VR applications consists of navigation tasks
which - in reality - include stimuli like airflow or
changes in temperature. Hence we suppose that an
extension of VR setups with wind and warmth will
improve users' state of presence. The hypothesis of
improved presence through multisensory stimuli is
supported by empirical work that shows the ability of
these modalities to do so (e.g. [DNH99], [CTV07],
[FW13]), although the visual sense is the most dominant
sense for humans (rods and cones inside the human
eye represent 70 percent of all human sensory cells
[CRT09]).

Only little work has been published in this field. There
is no standardized hardware nor well grounded concepts
for creating wind and warmth sensations and embedding
them in VR environments. Furthermore, experimental
results concerning the perception of wind and warmth
sensations in VR setups are rare.
This paper is an extension of the work first presented in
[HMFW13]. The concepts described in that paper are
summarized and extended by a detailed hardware and
software description followed by an extensive technical
evaluation. The steps conducted on the way toward a wind
and warmth simulator are the following:

Identification of requirements concerning wind
and warmth simulations in VR systems which
are independent from the applied VR setup
(e.g. CAVE vs. HMD)

Discussion of possible hardware and selection
of the most suitable system for the available
environment (three-sided CAVE)

Conception of an abstract model which embeds
wind and warmth in existing VR environments

Implementation of the software model and of the
hardware components in a suitable and efficient
way, keeping as much independence of precise
hardware and software environments as possible

Technical evaluation of the setup

Pilot studies gaining information on user
experience and presence

The paper is organized as follows: The second section
gives an overview of related work, dealing with wind and
warmth simulations in VR and their perception in humans.
The third section gives details of requirements on hardware
components and on the software implementation.
Afterwards a model for representing wind and warmth in
VR is given in Section 4. Section 5 describes the actual
implementation, starting with the hardware installation
followed by the software components. An evaluation
of the system is presented in Section 6 containing
a technical evaluation, and a pilot study analyzing
the influence of the simulation on user experience.
The last section summarizes the presented system
and outlines future work, projecting a more detailed
subjective evaluation of the system in terms of user
studies.

2.
State of the Art

As Dinh et al. [DNH99] have pointed out, the
introduction of multiple feedback modalities can enhance
the level of presence a user perceives in a VR environment.
Many systems have been developed, adding single
modalities to a graphics system. These have mostly
been auditive (e.g. [HMWK10], [LVK01]) or haptic
enrichments (e.g. [Bur96], [WSHP13]). Some efforts
including olfactory stimuli can be found, either using
global displays, which fill a whole room with odor, or
presenting scents via a wearable device. The main
problem using global approaches is the neutralization of
the submitted smell. One system minimizing this problem
is introduced by Yanagida et al. [YKN04]. Wearable
devices as presented in [YYT06] or [HIAI13], have the
drawback of being restrictive and are often regarded as
being too intrusive. Concerning wind and warmth
simulations, some approaches have been presented which
will be described in the following.

2.1.
Wind Simulation

One example for the integration of
wind is the VR-Scooter described by Deligiannidis and
Jacob [DJ06]. A real scooter is used for navigation inside
the VR and generates tactile feedback using vibrotactile
actuators. In addition, a fan provides wind in front of the
user. The authors detected that perceived wind improves
both, task performance and subjective user experience.
The main disadvantage of their system is the inability to
provide directional wind. Thus the system is only useful in
a rather narrow scope.

Moon and Kim introduced a setup which consists of 20
fans arranged in three horizontal levels around the
user [MK04]. A study showed that users cannot
distinguish between winds which come from neighboring
fans, if the angle in between is less than 45 degrees.
The system presented by Moon and Kim was able to
significantly enhance presence due to the application of
wind feedback. However, it was unable to compute
the wind influence in realtime: The authors used an
editor to design the wind field manually for each scene
beforehand.

In 2013, Verlinden et al. presented a sail simulator
which was extended by a wind simulation to increase
presence [VMV13]. Eight fans are mounted at a height of
two meters on top of the floor. All fans are arranged in a
circle with a diameter of four meters. The user is sitting
inside a simulator having the outward appearance
of a real boat. Graphics are projected on a screen in
front of the user. The system was evaluated with 10
test subjects. The results showed an improvement of
immersion when using the wind feedback. Furthermore
the participants reported that the wind simulation had
helped them to orient. On the other hand, most of them
perceived the wind sensations as being too weak. Also, the
simulation suffered from the sound created by the
fans.

Another approach is presented by Kulkarni et al.
who used the vortices of an airflow on the
projection wall to adapt the direction of a presented wind
up to an accuracy of 1-2 ° degrees [KMD12]. However,
the system can only be used with a certain display
arrangement and is thus not usable for a CAVE
environment. Also wind angles are only possible to be
perceived in the interval [-30 °, 30 °]. Furthermore
the wind cannot be created in realtime. A delay of 2 to 25 seconds appears when creating certain wind
directions.

In addition to the above mentioned stationary systems,
solutions directly mounted on the user's head are presented
e.g. by Lehmann et al. [LGWS09]. In their approach, a
comparison of stationary global and local systems
attached to the user was conducted: In a ski simulation,
three conditions (no wind, stationary fans, head-mounted
fans) were tested in a within-subjects study. The difference
between stationary fans and head-mounted fans was not
significant, but at least indicated a trend toward a higher
level of presence when using the stationary wind sources.
Further results showed a significantly higher level of
presence when using wind compared to the no-wind
condition.

2.2.
Warmth Simulation

Concerning the role of warmth in
VR, there is only little research. Dionisio published an
evaluation of possible hardware devices for temperature
sensations in 1996, concluding that fans, infrared lamps
and Peltier elements are the best option for VR-centered
temperature simulations [Dio97a]. An exemplary setup
was introduced as a "virtual hell" [Dio97b], with each
three fans and infrared lamps arranged around the user,
as well as Peltier elements attached directly to the
skin.

To find thresholds concerning the perception of heat and
cold, Gray et al. [GSM82] performed a study using Peltier
elements as temperature actuator. Thresholds were found
at changes of a temperature rise of 1.04 degrees Celsius
and a drop of 0.15 degrees Celsius: Interestingly, a
negative change in temperature is more easily perceived
than a positive change.

3.
Requirements to Simulate Wind and Warmth in Virtual
Reality

As only few projects combine wind and warmth
simulations with Virtual Reality setups, a detailed
investigation of necessary requirements is mandatory.
These should be divided into hardware and software
requirements being independent from each other. This
assures a maximal extensibility and generality of the
concepts. Furthermore, the requirements should be
independent from a specific implementation and also from
the existing setup (e.g. HMD vs. Cave and standardized
VR framework vs. self-developed system) in which a
wind and warmth system is integrated.

3.1.
Hardware Requirements

A first requirement is that the
system must be save: Especially when using heat sources,
neither the user nor the existing system itself must be
endangered. Therefore, a direct contact between user and
heat sources must be avoided. For instance the position of
heat sources mounted on top of the user has to be adapted
according to the user's height. Temperature limits
have to be selected carefully, because many hardware
components, e.g. projectors or the tracking system, are
heat-sensitive. Thus temperature sensors must be used to
monitor the overall heat created.

Further, the system must be capable of real-time
interactivity. A low latency toward the user's actions
(e.g. switching on/off a virtual fan or closing the coverage
of a virtual chimney) is required to apply the simulation in
a realistic Virtual Reality.

As the user is able to change translation and rotation
inside the virtual world, the simulation must adapt to the
transformation of a wind and warmth source continuously.
Otherwise, a gap between visual feedback and the
sensations perceived could appear and decrease the user's
sense of presence [WS98].

If the above mentioned criteria are satisfied, it must
be ensured that the user's interaction possibilities
are not limited by new hardware: Some hardware
solutions (e.g. concerning eyetracking [HDP11])
influence the user's behavior due to cables and hardware
attached directly to the user. Furthermore, the existing
hardware itself (e.g. projection or tracking) must not be
affected.

The following listing summarizes the proposed
hardware requirements arranged by priority:

3.2.
Software Requirements

Additional to the hardware
requirements, the software system must also satisfy
certain constraints. First of all, real-time capability must
be ensured: A direct reaction to the user's actions is
necessary in a Virtual Reality simulation. Otherwise,
the gap between visual information and wind and
warmth sensations could lead to decreased presence.
Altough, a realistic rendering is desirable, a physically
modeled simulation for calculating the exact influences
of wind and warmth sources would need too much
computational power. Also, it is doubtable whether such a
realistic simulation is necessary (e.g. for rendering light
and sound, simplified models like the Phong lighting
model are applied successfully). Therefore, a balance
between realism and computational costs has to be
found.

Inside the simulation, the user should be able to move
freely with six degrees of freedom. Therefore, the user
must be represented inside the simulation and the software
has to be able to adapt the sensation's direction and
position according to the user's actions. Furthermore,
virtual occlusions must be considered: If a user is placed
inside a virtual building or behind a wall, sensing the same
sensations as outside would be unnatural and thus disrupt
perceived presence inside the scene. The system must
react to a changed distance between wind/warmth source
and the user. Hence, an appropriate attenuation function
has to be found.

Wind and warmth sensations appear in different situations
and are created by different causations: airstream
created by the user's movement, storm belonging to the
simulation of a landscape, virtual fans, a fire inside a
rescue scenario et cetera. Therefore, an abstract model
which forms the basis for simulating a large set of
different sensation sources inside a VR framework has to
be found. To be able to integrate the system in an
existing Virtual Reality setup, also appropriate scales
must be found for quantifying the stimuli: This could
be for example m / s for wind or degrees Celsius for
warmth.

When using a combination of wind and warmth
sources, crossmodal influences could appear between
wind and warmth (as using wind also influences the
perceived temperature [Dio97a]) or wind-blown dispersal
at the projection walls. Such influences need to be
analyzed and considered when calculating the input signal
of the hardware system.

Last but not least, the system should be able to use the
full power of the hardware devices. The following listing
summarizes the software requirements arranged by
priority:

SW1 Provide real-time, low latency and a balance
between realism and computational costs

SW2 Adapt user's position and orientation inside the
virtual world

SW3 Consider occlusions

SW4 Use an appropriate attenuation function

SW5 Provide an abstract model to represent different
types of wind and heat sources

4.
A Model for Representing Wind and Warmth in
VR

A physically modeled simulation of wind and warmth
would be too complex to compute in real-time and with
low latency. The model presented here describes an
approximation for the application in a VR context. For
managing the VR scene, a scene graph is used. Inside the
scene graph, each object (e.g. a 3D model) and all further
entities (e.g. scripts controlling the application) are
represented as so-called nodes. Hence, each wind and
warmth source is also represented as a single node.
The VR framework sends these nodes and further
relevant parts of the scene to a dedicated sensation
engine. This engine calculates the influence of the
wind and warmth nodes on the scene and activates
the hardware components. Figure 1 illustrates this
process.

Figure 1. Short overview on the system's process flow. Parts of the InstantReality scenegraph (middle) is
transfered to the Sensation Engine (right) which activates the wind and warmth hardware (left) and informs
the scenegraph about changes in e.g. airflow.

Our concept to represent wind and warmth nodes is
inspired by the Phong lighting model and also by the
representation of sound objects inside Virtual Reality
applications first presented by Fröhlich and Wachsmuth
[FW12]. They present three kinds of sound: ambient
sound, static sound and event sound. Ambient sound can
be perceived in large areas inside a scene and does not
have a position. It is played with the same intensity on all
available speakers. Only one ambient sound can be
played at the same time. Static sounds are emitted
directly by an object to which they are attached. A
static sound's transformation is changed automatically
when the transformation of the corresponding object is
changed. The output of the individual speakers is
adapted to intensity and direction of the sound and to
the relative user position and rotation. Event sounds
are related to the static sounds, but only triggered
due to a certain event: This could be a ball hitting a
surface e.g. water, stone or a virtual door shut by a
user.

To simulate different types of sensations (e.g. fans, fire
et cetera), the following wind and warmth node types
were developed according to the above mentioned
concept:

Directional wind / directional warmth

Spot wind

Point warmth

The naming is inspired by the concept of representing light
sources in the X3D standard. The instances of the
nodetypes directional wind / directional
warmth have an infinite distance towards the user and are
analogous to the ambient sound nodes. Accordingly, they
are perceivable inside the whole scene from the same
direction (except from virtual occlusions). Examples for
the usage of these nodetypes could be weather influences
like storms or the effect of the sun. Nodes of the type
spot wind have a fixed position and are bound to
an object (e.g. to a virtual fan). These wind sources
only influence the area which lies in a directed cone
located in the wind source. The cone is defined by a
given angle and the intensity of the wind source (cf. figure 2). The nodetype point warmth is similar
to the spot wind: It is bound to a certain object
(e.g. a virtual chimney), but it has no direction. The
warmth is emitted toward all directions and therefore
the node is represented by a sphere. The radius of
the sphere is defined by the node's intensity. Both
nodetypes can be seen as analogous to static and event
sound.

Figure 2. Visualization of a spot wind produced by a
virtual fan.

5.
Implementing Wind and Warmth in a CAVE Environment

This chapter describes the actual implementation of the
wind and warmth simulation system according to the
requirements, starting with an overview of the existing VR
environment, followed by descriptions of the hardware
and software components.

5.1.
Basic Setup

The stimuli are presented inside a three-sided
(floor, front, left) CAVE environment. Six projectors use
polarized light for enabling stereoscopic vision. User
tracking is performed by a marker-based DTrack2 system
by ART, using 10 cameras. The cameras use infrared light
reflections for marker tracking. Spatial sound is enabled
by a 8.1 sound system: Eight active speakers are arranged
in a cube around the CAVE, a subwoofer system is embedded in
the CAVE floor. An air conditioner assures a constant
temperature of about 20 degrees Celsius. It does not
produce any wind which could be perceived inside the
CAVE.

To calculate the distributed graphical output, the
InstantReality framework by the Fraunhofer IGD is
applied. It is augmented by knowledge-based approaches
for managing and representing sound, haptics et cetera as
mentioned in [FW12]. The scene itself is represented
using an X3D-based scene graph.

5.2.
Hardware Implementation

Wind For the simulation of wind, the following
solutions were considered:

Accurate simulation of the wind direction using
an industrial fan and an approach as presented in
[KMD12]

CPU fans attached to the user as used in
[LGWS09], [CTV07] and similar approaches

In the following, the decision to use large scale fans attached
to the CAVE hardware is explained. Arguments are
followed by the related hardware requirements in
parentheses. An accurate simulation of the wind direction
as presented in [KMD12] is not possible, because of the
construction of the CAVE. Furthermore, their solution is
not real-time capable (HW2). While CPU fans are a cheap
solution and easy to control, they have to be attached
directly to the user to provide a sufficient wind simulation.
Hardware attached to the user could lead to a reduced
usability, because the user feels limited by the hardware
and may not act naturally anymore (HW4). The wind
created by an air conditioner is difficult to control
concerning the intensity, but also with regard to the wind
direction (HW3). Wind machines which could be
able to simulate a very realistic sensation of wind are
too noisy und would thus disrupt the user's presence
(HW4). Thus the only solution fully satisfying the
constraints is attaching large scale fans directly to the
CAVE: Eight fans by ADDA are attached in a circle
above the projection walls with an angle of 45 degrees
in between. The frontal fans could not be placed as
optimal as the other fans for technical reasons: The
back-projection mirror for the floor needs too much free
space.

Finally, due to the angle approximating the 45 degrees
proposed by Moon et al. [MK04], the wind direction can
be simulated continuously on the horizontal plane (HW3).
A direct contact between fans and user or other parts
of the hardware is prevented by their positioning.
Therefore, endangering user or hardware can be excluded
(HW1). Each fan has a diameter of 25.4 cm and a wind
performance of 12.735 m³ / min. All fans are operated at
115 instead of 230 Volt to reduce noise (HW4): From a
distance of one meter, a noise volume of 55.6 dB/A can
be measured. Figure 3 shows a single fan and the
mounting of the hardware inside the CAVE. Figure 4
details the overall hardware architecture. Table 1
summarizes the technical data. All fans are controlled in
real-time via MultiDimMKIII Dimmerpacks by ShowTec
(HW2).

Table 1. Axialfan: Detailed technical data.

Fan Specification

Device Name

Axialfan AK25489 by ADDA

Emitted Noise

55.6 dB/A

Power

24.2 W

Voltage

115 V

Shielding Angle (one direction)

approx. 35 °

Maximum Intensity

12.735 m³ / min

Diameter

25.4 cm

Weight

2.00 kg

Cost

60 Euro

Warmth According to Dionisio [Dio97a] the following
hardware solutions were considered:

Heater blower

Infrared lamps

Peltier elements

Radiant heater

Air conditioner

Electric blankets/suits

In the following, the decision to use infrared lamps is
explained similar to the argumentation concerning the
wind hardware. A radiant heater would - because of the
usage of gas fuel - be too dangerous for the usage in the
VR lab (HW1). The air conditioner was excluded because
of its inertness, not being capable of real-time heating and
because it requires a complex control mechanism without
standardized interfaces (HW2). For most radiant heaters,
the temperature cannot be set continuously (HW3).
Furthermore, they need a time range of about one minute
to reach an adequate temperature and thus are not
real-time capable (HW2). In related projects, Peltier
elements are used [Dio97a]. However, they must be
attached directly to the user. This would raise the
risk to reduce the system's usability and thus also the
perceivable presence (HW4). Electric blankets or
warming suits were also excluded, because like the
Peltier elements, they could lead to a reduced usability
(HW4).

Finally, only infrared lamps appear to be usable for
warmth simulations inside a CAVE: They endanger (if the
overall temperature is controlled by sensors) neither
user nor hardware (HW1) and react instantly (HW2).
Nevertheless, a system consisting of one infrared lamp
would be unable to simulate spatial warmth (HW3).
Furthermore, the warmth intensity created by a single
lamp using standard E27 sockets is not sufficient.
Therefore, a system consisting of six infrared lamps
placed on top of the CAVE is selected. Each lamp
has a power of 250 Watt and is able to heat the area
directly around the lamp to up to 100 degrees Celsius.
All lamps are placed on top of the CAVE to prevent
a distraction from the hardware setup (HW4) and
because the user's head is, according to [KHK09], one of
the most sensitive areas for warmth stimuli (most
other parts of the body are covered with clothes).
The lamps distance toward the user can be varied
according to the user's height (HW1). To prevent the
system from getting too hot, two M-ware® temperature
sensors are attached to the CAVE (HW1). Unfortunately
the infrared lamps emit some visible light, which
distorts the projection. Therefore heat-proof color
foils are attached in front of the lamps; sideward,
they are surrounded by metal (HW4). If a lamp is
directed toward a tracking camera, a part of its field
of view is distorted. These areas are automatically
determined and excluded from tracking. Thus the
appearance of false positives when switching on the
lamps is prohibited. A part of the technical evaluation
presented in Section 6.1.1 shows that tracking is not
affected by the usage of the lamps (HW4). Like the fans,
the infrared lamps are also controlled in real-time by
MultiDimMKIII Dimmerpacks (HW2). Figure 3 shows
a single infrared lamp with the heat-proof color foil, the metal surrounding and the mounting inside the
CAVE.

Figure 3.
Axialfan AK25489 by ADDA and infrared lamp with color foils and metal surrounding mounted on
top of the CAVE with the projection switched on.

Figure 3 shows the mounting of fans and infrared
lamps inside the CAVE. Figure 4 gives an overview of
the whole setup containing the hardware for creating wind
and warmth.

Figure 4. Overview of the CAVE environment enriched with fans and infrared lamps.

Table 2. Detailed technical data of a single infrared
lamp.

Lamp Specification

Device Name

Osram THERA R126 Red

Emitted Noise

not noticeable

Power

250 W

Voltage

240 V

Shielding Angle (one direction)

30 °

Maximum Intensity

100 °C

Diameter

15 cm (incl. surrouding)

Weight

0.12 kg

Cost

10 Euro

5.3.
Software Implementation

The software implementation
consists of three main components: A wrapper for the
above mentioned wind and warmth sources represented in
the Virtual Reality framework (InstantReality), the
sensation engine and a library loaded to establish the
connection between VR framework and sensation engine.
The library loaded by the VR framework sends all
necessary changes inside the scene to the sensation
engine. This is performed in fixed intervals. Afterwards,
the engine calculates the activation of the wind and
warmth hardware and propagates changes throughout the
VR framework. Figure 5 describes the information flow
between the components of the wind and warmth
framework.

Figure 5. Information flow between the components in
the wind and warmth framework as part of the VR
framework.

The implementation of the sensation sources inside the
VR framework consists of one wrapper node for each
wind and warmth nodetype: directional wind, directional
warmth, spot wind and point warmth. Furthermore,
physical objects which are able to react to changes of the
wind intensity (e.g. clouds, falling leaves, flags) are
enriched via a connection to the sensation engine:
The engine informs them about the airflow at their
current position. The detection of virtual occlusions is
implemented inside the graphics framework: Transfering
all necessary geometries for occlusion checks to the
sensation engine, would create unnecessary load for the
network connection. For each object influenceable by
wind effects and for the user himself, occlusions are
checked as described in Algorithm 1.

Algorithm 1. Occlusion check. The constant
MAX_DISTANCE ensures that objects like trees
or small hills which are too far away from the
user can be ignored for occlusion. d is the distance
between the position of the object for which the wind
influence is tested and the point which is hit by the
ray.

The library establishing the link between VR framework
and sensation engine acts as a parser for the node format
used in the VR framework. The nodes are serialized for
the sensation engine and the information provided
by the engine is transformed into valid nodetypes
of the corresponding VR framework. This approach
allows the usage of different VR/Graphics frameworks,
e.g. InstantReality, Unity, Unreal Engine, Blender et
cetera. Inside the sensation engine, the following steps are
executed:

Update all nodes received from the client

Execute calculations for wind and warmth effects

Activate hardware and update the connected VR
framework about wind and warmth influences on
the scene

Get information from temperature sensors for
preventing overheating

The calculations to activate the hardware are described
in algorithm 2: The activation for each hardware
component has to be calculated for each sensation node
via using the angle between
(vector from sensation
node to user position) and
(vector
from hardware component to user). Afterwards all
activations for each hardware component are summed up
and normalized. Here, it is assumed that wind and
warmth sensations which have an influence on each
other accumulate additive which was suggested by
preceding tests. The hardware is controlled via the DMX
protocol.

Algorithm 2. Mapping of sensation nodes on the
hardware components satisfying requirement (SW2)
and (SW8). Here, HWComponents stands for
the fans and infrared lamps. HWDirection is
the direction of the mounted hardware component
toward the user.

6.
Evaluation

The system installed at Bielefeld University
was evaluated with respect to technical properties
of the hardware and software parts. Furthermore, a
pilot study indicated a high level of presence and user
experience which could be reached by the system. This
section first presents technical evaluations followed by
two user studies. The first one analyzes the accuracy
with which the wind direction can be estimated by
users, the reaction times regarding wind and warmth
stimuli and the minimal activation of the hardware
which is necessary to have any effect on the user.
The second user study analyzes the subjective user
experience.

6.1.
Analysis of the Technical Setup

To evaluate the
technical setup objectively, the following questions have to
be answered:

Do the infrared heat lamps interfere with the
infrared-based tracking system?

How much latency does the system have and how
many wind and warmth sources can be added to
a particular scene without reducing the overall
performance?

What is the size of the volume influenced by
the wind and warmth hardware and what are the
possible maximum values for wind speed and
temperature?

6.1.1.
Influence of the infrared lamps on marker-based
tracking

As the tracking system uses infrared light
reflections to determine the position of the markers, the
tracking performance could suffer from the use of the
infrared lamps. To analyze the possible influence of the
lamps, a tool measuring the size of the tracking volume
has been developed and works as follows:

Move a tracking target manually through the
CAVE. If one of the cuboids is hit at least twice
by the target, mark it it as covered by the tracking

Return the percentage and the positions of
covered cuboids

First, the coverage of the original setup (without the use of
infrared lamps) is determined. Afterwards, all infrared
lamps are switched on with full power. Some of the
tracking cameras are directly spotted by the lamps. This
leads to some areas in the lens coverage of single cameras
which are not usable for tracking. These areas are
automatically excluded in a calibration pass. Figure 6
visualizes the areas excluded for single cameras. The
target is then again moved through the CAVE. Afterwards,
the results with and without infrared lamps can be
compared.

Figure 6. Each box displays the analyzed image for a
single tracking camera. The dark areas are excluded
from tracking, because they are directly spotted by the
infrared lamps.

The coverage was measured for three kinds of targets: A
worn passive rigid body with strong signs of usage, a
new passive rigid body, and an active target used for
handtracking.

Table 3. Percentage of covered tracking volume for
three kinds of targets. Results were the same with and
without infrared lamps.

Target

Coverage

Worn passive

55 %

New passive

96 %

Active

81 %

The tracking volume was not influenced in any of the
tests. Table 3 shows the results which were the same for
both tests. Yet it must be suspected that in a different
setting (e.g. with more infrared lamps or with fewer
tracking cameras), the tracking could be affected by the
lamps. In these cases, the lens coverage directly disturbed
by the lamps might be too large which would make it
impossible to exclude it from tracking without reducing
the trackable volume. Nevertheless, using enough tracking
cameras (10 in our case), the overall quality of the
tracking is not influenced by the use of the infrared
lamps.

6.1.2.
Performance

The sensation simulator runs at about
30 fps. To determine the time, the wind and warmth
actuators need to influence the area at the center of the
CAVE, the following experiment was conducted: The
hardware components are activated as if a virtual wind or
warmth source would be presented toward a user. In the
center of the CAVE, a device to measure the stimulus is
placed inside the volume the user's head normally fills.
Afterwards, the wind node is activated and the time after
the measurement device detects a change in wind speed is
taken. A similar experiment was conducted to measure the
reaction time of the lamps. After determining the time, the
lamps and fans need to be switched on and make their
sensations perceivable inside the CAVE. The time to
deactivate the stimuli is measured using a similar
procedure.

To minimize the delay the measurement device
for wind speed induces, it was replaced by a simple
pendulum trackable by the ART tracking system. The time
the pendulum needed to be affected by the wind was
measured. For measuring the warmth, a VOLTCRAFT
IR 280 measurement device was used (latency < 0.5 s).

Warmth needed 1.5 seconds to influence the measured
area after the lamps were switched on and 1.8 seconds to
create a measurable temperature difference after switching
the lamps off. The values for wind were 2.1 seconds for
switching on and 1.1 seconds for switching off. The
hardware itself reacts instantly, the measured values
represent the time which the air at the center of the CAVE
needs to be moved by the fans or to be heated by the
lamps.

In a second step, the number of possible wind and
warmth sources in a typical VR scene was tested: A
number of 150 to 200 wind or warmth sources can be used
simultaneously without reducing the framerate. As for
typical VR scenes, not more than 10 or 20 different stimuli
sources are used, the wind and warmth system does not
affect the overall simulation performance.

6.1.3.
Intensity distribution of wind and warmth

To determine
the intensity distribution of the presented stimuli, the
intensity of a single wind and a single warmth source was
measured on 20 points equally distributed on a horizontal
grid inside the CAVE. As intensity measure, meters per
second (m / s) were used to describe the wind intensity,
and degrees Celsius were used to describe the warmth
intensity. A wind measurement device by ELV was
used to determine the wind speed. As temperature
sensor, the VOLTCRAFT IR 280 was used. Figure
7 shows the grid and the infrared lamps and fans
used to simulate the virtual wind and warmth source:
Both stimuli have their origin directly in front of the
typical user position. The size of the hardware devices
shown in the figure relatively depicts their activation
level.

Figure 7. Grid to measure the distribution of wind
and warmth inside the CAVE. The pictured hardware
devices' size relatively depicts their activation
level. Each box inside the graphic visualizes one
measurement point.

The warmth distribution was investigated on three
planes with a vertical distance of 25 cm, 35 cm and 45 cm
toward the lamps. If all infrared lamps are switched off,
the room has a basic temperature of about 20 °C. This
temperature is normally assured by two air conditioners.
However, the lamps are able to heat the room temporarily,
because of the slow reaction time of the air conditioners.
After activating the simulation of a virtual warmth
source directly behind the front wall of the CAVE, the
highest temperature measured on the grid was 28.7 °C,
which is a possible temperature increase of 8.5 °C.
The lowest temperature measured on the grid was
22.6 °C. Figure 8 shows the warmth distribution on the
grid for all three planes. The results indicate that a
high level of temperature increase is possible (7.1 °C more than the threshold determined by Gray et al. to
make warmth perceivable on the skin [GSM82]). The
temperature is distributed quite constantly inside the
whole CAVE area. Therefore it is assumed that the warmth
direction cannot be perceived by possible users in a solid
way.

The wind distribution is measured on 5 planes with a
vertical distance of 45 cm, 55 cm, 65 cm, 75 cm and 85 cm toward the fans. Without activating any virtual
wind source, no wind flow can be measured inside
the whole area. After activating the simulation of a
virtual wind source directly behind the front wall, a
maximum wind speed of 19.7 m / s can be measured. It is
eye-catching that the wind intensity strongly depends on
the position and direction of the fans. For this evaluation a
frontal wind source was used, because this represents
the worst case concerning the wind distribution: The
frontal fans are not placed as equal as the other fans
inside the setup as explained in Section 3.1. The
results (see Figure 9) suggest that the best area to
perceive wind sources from arbitrary directions is
in the center of the CAVE. Although the influence
is quite moderate, the results for different vertical
distances are balanced and a wind speed of 2.3 m / s
on average (45 cm: 3.6 m / s, 55 cm: 4.4 m / s, 65 cm:
3.6 m / s, 75 and 85 cm: 0 m / s - too little wind to be
measurable) can be measured in the center of the CAVE.
Furthermore, an equal wind quality for most possible wind
directions is expected for this central position which is
also the most typical position of the user in many
applications.

Figure 9. Distribution of wind inside the CAVE
depending on the vertical distance toward the fans.

6.2.
Wind Direction, Reaction Times and Activation
Thresholds

In a user-centered evaluation, the following
questions were investigated:

How precisely can the direction
of presented virtual wind sources
be estimated by test subjects?
[1]

What is users' reaction time toward the activation
and deactivation of presented wind and warmth
stimuli?

How much activation must be at least provided
by the fans and the infrared lamps to make the
stimulus perceivable?

To answer these questions, a small study with N = 9 subjects
was conducted. Their age ranged between 20 and 51 years
(M = 22.67, SD = 10.3). They were recruited through
postings in the university building and all of them were
native speakers of German. Most were students at
Bielefeld University, three were female. The design
comprises three steps with the same participants. First, a
set of given wind directions had to be estimated by the
participants. A Nintendo Wii remote was used to point at
the estimated wind direction. Second, the reaction times
were evaluated, and third the activation thresholds
were tested. To reduce hints on possible activations of
hardware components, participants were blindfolded
in each stage of the experiment in which wind was
used. Additionally, an artificial wind sound was faded
in - independently from the actual activation of the
hardware - to prevent participants from hearing the
fans.

6.2.1.
Setup

Before starting the experiment, the participant
was placed in the center of the CAVE. The Nintendo Wii
remote equipped with a tracking target was placed in the
dominant hand. At first, the accuracy for determining wind
directions was tested. As pointing with the Wii remote
could be error-prone and because not all possible
participants could be expected to have the same pointing
accuracy using this device, the accuracy was evaluated
before the actual experiment as follows: A blue circle was
projected directly on one of the CAVE screens without
using the system's 3D mode. The participant used the Wii
remote to point at the circle and pressed a button. This was
repeated eight times and the pointing error with respect to
x and z axis was saved. The positions of the circles were
varied between participants using a 9x9 Latin square
pattern.

Directly before starting the actual experiment,
the participant was blindfolded. Then the visual
stimuli were replaced by wind stimuli. Again, the
participant used the Wii remote to point toward the wind
source. Then a button was pressed and the wind was
deactivated. After a short period, a new wind source
was presented. This was repeated for eight times.
Now, the error with respect to the x and z axis was
determined [2].
The possible wind positions were varied by using a 9x9
Latin square pattern. They were arranged all around the
user to ensure a good coverage.

The reaction times toward presented stimuli were
tested during a second pass: The participant was still
blindfolded and a virtual wind source directly in front of
the participant was activated on full power. As soon as the
participant perceived the stimulus, a button on the Wii
remote had to be pressed. The stimulus was then presented
for a few more seconds and finally switched off. As soon
as the participant had perceived that the stimulus had
stopped, the button had to be pressed again. Afterwards,
the same procedure was executed using a warmth
source.

In a third step, hardware activation thresholds necessary
to make a stimulus perceivable were determined: Fans
were activated in five percent steps until the participant
perceived the stimulus and pressed the button. This was
repeated using the infrared lamps afterwards. Here,
the lamps and fans were not switched off during the
on phases, because the temperature threshold which
is necessary to perceive a change in temperature, is
smaller if the temperature decreases [GSM82]. This
would have led to a falsification of the results: The
participants would have reacted to the switch off events
and not as it was desired toward the increase of the
temperature.

6.2.2.
Procedure

After entering the laboratory, the participant
filled in a consent form and was accompanied to the
CAVE. There, demographic data was collected during a
short interview conducted by the investigator. Then the
participant, equipped with tracked 3D goggles and a Wii
remote, was placed in the center of the CAVE according to
the results found in chapter 6.1.3. The investigator who
led the experiment blindfolded and unblindfolded the
participant and gave the instructions for each part of the
three experiment passes. The whole procedure lasted
approximately 10 minutes.

6.2.3.
Results and discussion

The accuracy to point on a
presented wind source has a mean angle of M = 14.13
degrees (SD = 30.25) after subtracting the error to point
with the Wii remote. The results are visualized in figure
10.

Figure 10. Accuracy concerning the detection of the
wind direction. The small white circle visualizes the
user.

Regarding the reaction time, typical users need to
react toward a wind stimulus, the time to perceive the
appearance of the stimulus is significantly longer (p < 0.01) than the time for perceiving the disapperance
(M+ = 3.1 s, M- = 1.3 s,). Concerning warmth, there is
no significant difference (M+ = 2.5 s, M- = 2.0 s,).
Table 4 summarizes these results. Altogether, the
reaction times suffice to simulate static and ambient
wind sources, but also event winds for e.g. a passing
train or the approaching toward a warm chimney in
realtime. Shorter events e.g. the airflow created by a door
which is closed, can be simulated in many cases by
precomputing the time, at which the wind stimulus should
start.

Table 4. Reaction times of participants toward
activation and deactivation of wind and heat sources.

Reaction Time

Time in seconds: Mean (SD)

Fans on

3.144 (0.290)

Fans off

1.313 (0.850)

Lamps on

2.450 (1.933)

Lamps off

2.045 (2.081)

For the activation thresholds which are at minimum
necessary to make a user perceive a stimulus, the
following results were found: Warmth is detected with an
activation of M = 57 percent (SD = 21). For wind, the
values are slightly lower with M = 47 percent (SD = 9).
These results must be considered when developing
applications using the presented system. Furthermore, the
high values suggest the use of stronger hardware devices if
providing a more intense feeling is necessary (e.g. for
firefighter simulations).

6.3.
Pilot Study on User Experience

To gain a first hint on
the user experience, which can be reached using the
presented system, a pilot study with N = 23 participants (9
female, 14 male, Mage = 25 years, SDage = 8.68) was
conducted. The participants were mainly students
recruited in the university building. All of them were
randomly assigned to one of two between-subject
conditions: The control group (N=12) experiences certain
Virtual Reality scenes without wind and warmth stimuli,
the test group (N=11) perceives the scenes enriched with
wind and warmth.

6.3.1.
Setup

The participant wearing 3D goggles and a
Nintendo Wii remote was placed inside the CAVE. An
information text was projected on the front wall and
described the participant's fictitious role in the following
experiment: "As a researcher on an expedition you will
move through certain different environments. During the
experiment, you have to solve small tasks".

Subsequently, another text was presented. It contained a
short introduction to the following virtual world and
instructed the participant to closely investigate and
remember the environment. Now the first scene was
presented. To prevent overcharging the participant with
learning special navigation techniques, the participant was
moved automatically through the environment. The only
direct interaction with the scene consisted of virtual
buttons which were able to be activated by the participant
using the Wii remote. After moving through the scene, a questionnaire was filled out. This procedure was repeated
with five different scenes. To prevent an influence on the
results according to the order in which the scenes were
presented, they were varied using a 5x5 Latin square
pattern. Each scene contained wind and/or warmth stimuli.
The following list presents the scenes used (cf. figure
11 ):

Desert (Warmth) The room in
which the participant is placed has a gate at one
side. It can be opened by using a virtual button.
As soon as the gate is opened, the participant is
moved through a sparse desert-like environment.
When the participant enters the desert, a warmth
stimulus is presented.

Volcano (Wind and Warmth) The participant is
placed on a scaffold over an ocean. The scaffold
leads to an isle consisting of a volcano. At first -
when being on the scaffold - a wind stimulus can
be perceived. When approaching the volcano's
lava lake, the wind is occluded by a rock and a
warmth stimulus is presented.

Fan (Wind) A living room is crossed by the
participant. From the right, a wind stimulus can
be perceived. It results from a virtual fan, which
is placed on a desk. The participant can control
the fan's intensity by using virtual buttons on the
desk.

Chimney (Warmth) In a small room, a virtual
chimney is presented. First, it is covered by a
transparent pane. The participant can remove the
pane by pressing a virtual button. This intensifies
the presented warmth stimulus.

Train (Wind) The participant moves through a
railway tunnel. After a time, sounds created
by an approaching train can be perceived. The
navigation speed is increased and just before
the train arrives, the participant is stopped and
placed next to the tunnels wall. When passing the
participant, the train produces wind.

Figure 11. Scenes presented for the subjective measures. From left to right and top to bottom: Desert, Volcano,
Fan, Chimney and Train.

6.3.2.
Measures

The following dependent variables were
analyzed to gain a hint on user experience:

Realism of the intensity of wind and warmth

Realism of the direction of wind and warmth

Perceived presence

The dependent variables were analyzed and measured
using five types of questionnaires: (1) One questionnaire
was used for surveying demographic information. (2) The
standardized Immersion Tendency Questionnaire (ITQ)
developed by Witmer and Singer [WS98] consists of 12
questions and measures the capability of individuals to
get immersed in everyday activities (e.g. reading,
watching movies). (3) The standardized Simulator
Sickness Questionnaire [KLBL93] measures simulator
sickness induced by the virtual environment. (4) A
modified version of the Slater, Usoh, Stead Questionnaire
(SUS) [UCAS00] was applied to provide a measure for
determining the perceived presence during the experiment
(in the interval [0,9]). (5) A further questionnaire
adapted to the scenes determined the quality of the
stimuli's direction and intensity and includes further
open questions about the participants' experience.
The realism of the intensity was rated on a five point
scale in the interval [-2,+2], where zero is the optimal
value. Lower values stand for weak stimuli, higher
values for stimuli which are too intense. The direction
was also rated on a five point scale, in this case in
the interval [0,+4]. Here zero stands for the optimal
direction and higher values indicate a less realistic
perceived wind direction compared with the visual
stimuli.

6.3.3. Procedure

Before entering the virtual environment,
the participant had to fill in a consent form and the
questionnaires 1, 2 and 3. Then, the participant was placed
in the center of the CAVE and equipped with a Wii remote
and tracked goggles. After explaining the procedure,
the information text was presented on the projection
walls. Next, each scene was presented and followed by
questionnaire 4 and 5. After encountering all five scenes,
the Simulator Sickness Questionnaire was answered again.
Eventually, the participant was thanked for joining the
experiment. This procedure needed approximately 30
minutes.

6.3.4.
Results and discussion

Both groups had a similar
immersion tendency as measured by the ITQ (Mcontrol = 31,
SDcontrol = 11, Mww = 36, SDww = 12). Figure 12
visualizes the perceived quality of the stimuli's direction
and the intensity. See table 5 and table 6 for the results
broken down for the single scenes. The intensity is rated
as realistic, but in some cases slightly too weak for both
types of stimuli. There is no significant difference between
wind and warmth. The results for the direction are similar,
here are also no significant differences between wind and
warmth.

Figure 12. Quality of the perceived direction and
intensity of the stimulus for group WW (0: optimal
value).

Looking closer at the scenes, it becomes noticeable that
in only one scene (Scene 3: Fan), the stimulus intensity is
rated as being too strong compared with the other scenes
(M = 0.273, SD = 1.0, p < 0.05). Furthermore, the
results concerning the wind direction indicate a trend to
weaker results in this scene than to all other scenes (M
= 1.545, SD = 1.2, p = 0.1). This finding could be
caused by a gap between visual and wind stimuli: The
participant perceived the wind before being able to
recognize the virtual fan. So there is no explanation for the
sudden appearance of the wind. Therefore, the overall
realism of the stimulus is rated worse, because the
visual stimulus is more important when perceiving the
scene.

For Scene 4 (Chimney), slightly worse results concerning
the intensity (M = -1.182, SD=0.75, p = 0.618) were
reported. Two reasons seem possible: (1) The overall
intensity is too low compared with other scenes or (2) the
visual direction of the stimulus contrasts with the
perceived direction (visual direction: from the front,
position of the infrared lamps: on top of the user). Both
hypothesis can neither be verified nor falsified: The fire's
intensity is comparable with the stimulus intensities found
in the other scenes. Nevertheless, the distance toward the
virtual warmth source is slightly greater than the distance
implemented in the other scenes. For verifying the second
hypothesis, the results concerning the direction should
be worse for Scene 4 (Chimney). The mean values
are in fact slightly different, but these results are not
significant. Thats why further studies should evaluate a
necessary analogy between visual and wind/warmth
stimuli.

Concerning the perceived presence, no significant
results were recognized as expected due to the small
number of participants. Nevertheless, a trend can be
observed for Scene 1 (Desert) and Scene 2 (Volcano): Here,
the results are slightly better when using wind and
warmth (MdesertWW = 4.9, MdesertControl = 3.4,
pdesert = 0.086, MvolcanoWW = 6.5, MvolcanoControl = 5.3,
pvolcano = 0.13). See figure 13 for a visualization of the
results.

Finally, our results indicate a high level of user
experience and enforce the assumption that the realized
wind and warmth simulation is able to enhance the
perceivable presence. These results suggest further
investigations and more detailed evaluations.

7.
Conclusion and Future Work

This paper described a setup to create wind and warmth
simulations in a Virtual Reality environment. First,
general requirements to simulate wind and warmth
in the context of VR were developed and analyzed.
Further, possible hardware devices to produce the
stimuli were discussed and the best hardware for the
described CAVE environment was chosen. Afterwards, the
setup of the hardware was described with respect to the
requirements. A software model to represent the stimuli
inside the VR framework and for the communication
between VR framework and hardware was developed.
Furthermore, the system was evaluated with respect to
technical details, namely possible disturbances of
hardware, latency and real-time aspects, hardware
performance and quality of the framework to determine
direction and intensity of the stimuli. Finally, a pilot study
suggests a rise in user experience and also a trend
concerning a rise of perceivable presence which could be
realized by the system. The presented system is - to our
knowledge - one of the most extensive, most flexible and
best evaluated systems to create wind and warmth
stimuli in Virtual Reality applications designed for
CAVE environments. Furthermore, the low cost system
(approx. 2200 Euro) is not only applicable for the
described CAVE environment, but also for example
for HMD setups. Table 7 summarizes the system's
properties.

Table 7. Overview of wind and warmth system.

Wind System

Warmth System

Number of devices

8

6

Height of mounting

2.5 m

2.3 m (during tech. mesurements, variable according to user's height)

Covered area

3 m x 3 m

3 m x 3 m

Max. virtual sources without influencing performance

150-200

150-200

Cost additional hardware

250 Euro (adapted DMX Dimpacks)

250 Euro (DMX Dimpacks and temperature sensors)

Cost mounting

390 Euro

800 Euro (incl. additional traverses)

Total cost

1120 Euro

1110 Euro

Although the system's capabilities are satisfactory and
the reported results are promising, there is a large potential
for possible further research. Two areas are in the center of
future tasks: Improvements of the system itself and more
detailed large-scale evaluations.

Starting with system improvements, virtual occlusions
are handled quite rudimentarily: A stimulus is occluded
or it is not. But what about for example concerning
the difference between a fire which is occluded by a
thick concrete wall (e.g. a tunnel wall) and a fire
which is occluded simply by a pane of glass (e.g. a
window of a car)? To be able to simulate these situations,
materials must be equipped with a kind of wind/warmth
transparency factor alpha. To handle the occlusions itself,
algorithms well known from the area of computer graphics
(e.g. shadowing) could be used. At the moment, warmth
sources are only bound to one single sphere. Binding
them to more complex objects, e.g. arbitrary meshes
would make the design of complex scenes, e.g. a
warm pipeline or the warmth of a virtual large scale
fire.

Concerning the user experience and especially the
achieved level of presence, the presented results are
quite promising. Nevertheless, a detailed study with
more participants and more detailed questionnaires is
necessary. Furthermore, measurement of heart rate and
skin conductance as applied by Slater et al. in [SGE06] is
desirable for quantifying the perceived presence and for
gaining significant results. Here, it is planned to analyze
how necessary a close correlation of wind/warmth stimuli
and the visual output is. How closely need wind and
warmth stimuli to be correlated with visual stimuli?
How does presence decrease in relation to increased
distance between wind and warmth and the correlated
visual output? Also, a determination of crossmodal
influences between wind and warmth stimuli presented
simultaneously would seem interesting. We believe that
further advancement in this direction will make wind and
temperature sensations as common in future Virtual
Reality as sound is today.

8.
Acknowledgments

The authors would like to thank
Nico Lüdike for his technical work in preparing our
previous approach for the wind system.

[1] The accuracy concerning the direction of a warmth source
does not need to be analyzed, because the warmth is too equally
distributed inside the CAVE area to promise worthwile results (see
section 6.1.3).

[2] The used wind hardware is unable to simulate any change on the y
axis, therefore this axis was excluded.